During the last seven years my research group at Illinois has concerned itself with the problem of biosynthesis, biomagnification, and neurotoxicity of methylmercury. We understand B12-dependent methylmercury synthesis, and have discovered a chemical basis for methylmercury neurotoxicity. We are presently studying the role of B12 in selenium salt detoxification of methylmercury poisoned organisms. In addition we have pioneered research on the mechanism of B12-transport from the gastro-intestinal tract into the bloodstream. In the final year of this grant Drs. Cheh and Segall will turn attention to principles of B12-transport, and B12-dependent detoxification reactions. Mr. Frick will continue his work on the mechanism of alkyl-transfer form B12 to metals (toxic).